Generally, a cooling system for an engine in a vehicle has a cooling-water circulation passage with a thermostat. When cooling water is low-temperature at cold startup, the thermostat operates to close the cooling-water circulating passage between an engine and a radiator and open a water passage for returning of the cooling water from the engine to the engine not by way of the radiator, thereby preferentially warm up the engine through circulation of the cooling water not by way of the radiator.
As is well-known in the art, in this kind of thermostat, wax filled in a casing melts as the cooling water becomes high-temperature; and expansion of the wax due to the melting thereof causes a valve to open through a needle, a spring or the like. The operation is conducted on the basis of a mechanical principle.
As a prior art literature pertinent to this kind of cooling system for an engine, there already exists, for example, the following Patent Literature 1.